1. Field of the Invention
The present invention relates generally to high-load centralizer systems and, more specifically, provides a system for securing one or more centralizers to a pipe coated with elastomeric or other suitable insulation material which in one preferred embodiment may be utilized as non-fixed riser interconnection or stress pipe riser interconnection between a floating platform and/or a subsea wellbore.
2. Description of the Prior Art
Marine risers have been utilized in the past with non-fixed riser interconnections, also referred to herein sometimes as stress joints and/or keel joints, which provide riser interconnections to floating platforms and/or drill ships and/or wellheads. A stress joint utilized adjacent the floating platform may sometimes be referred to herein as a keel joint because it extends through the bottom or the keel of the platform or other marine vessel. As used herein floating platform may refer to any marine structure. The floating platforms are generally maintained above the wellhead, or in the general vicinity of a plurality of wellheads. One type of non-fixed connection for this purpose is described in some detail in U.S. Pat. No. 4,185,694, issued Jan. 29, 1980, to E. E. Horton, which is incorporated herein by reference. More specifically U.S. Pat. No. 4,185,694 provides for a marine riser system which extends between a floating offshore platform and one or more well means in a seabed formation and which has riser end portions non-fixedly connected in to the floating platform and to wellhead structure at the well hole. Each end portion of the riser may be adapted to yield axially, laterally, and rotatively during movement of the riser relative to the platform and to the wellhead structure. Each end portion of the riser is provided with fulcrum or pivot contacts, which may preferably comprise centralizers, with hawse pipe carried by the platform and with hawse pipe or casing means provided in the wellhead structure. Bending stresses at the riser end portions or stress joints are reduced at the platform and at the wellhead structure by utilizing the non-fixed connection described in greater detail therein.
The non-fixed connections, which may also be referred to herein as stress joints, utilize centralizers which are subject to considerable forces, as the centralizers act as fulcrums as described in the above cited U.S. Pat. No. 4,185,694. Due to the high stresses involved, over time undesirable axial slippage of the centralizers may occur with respect to the non-fixed riser connection or stress joint. Other problems with the non-fixed connections that may exacerbate the slippage of the centralizer include corrosion and/or galvanic action between for instance, the wellhead and the non-fixed riser connection or stress joint wherein the wellhead and centralizers may comprise steel and the stress joint may comprise titanium and/or steel.
U.S. Pat. No. 5,887,659, issued Mar. 30, 1999, to B. J. Watkins, discloses an assembly including a protective sleeve spaced about an intermediate pipe of a riser which is adapted to extend through an opening in the bottom of a vertical compartment of a offshore rig for use in drilling or completing a subsea well, with a ball shaped portion on the upper end of the sleeve is closely received by ball shaped surfaces of the upper portion of the riser pipe, while a ball shaped part on the lower portion of the riser pipe is so received within the lower end of the sleeve to permit them to swivel as well as to move vertically with respect to one another.
Other attempts to reduce minimize or distribute forces applied to stress joints and/or keel joints are shown in the following documents:
U.S. Pat. No. 6,422,791, issued Jul. 23, 2002, to Pallini, Jr. et al., discloses an attachment which extends between an outer sleeve and an inner riser pipe where the pipe penetrates the keel of a platform. In one version, the attachment is a conically-shaped with a small diameter ring that engages the riser pipe and a large diameter ring that engages the outer sleeve. This attachment has elements that are very flexible in bending but relatively stiff and strong in axial load. Other versions include flat rings where lateral load is taken directly into tension and compression in the beams, allowing for relatively high lateral load transfer. Both the conically-shaped attachment and the flat ring have a number of variations that provide low bending stiffness but high axial stiffness of the elements. Depending on whether resistance to axial loads, lateral loads, or resistance to combination of both loads is desired, the attachment and the flat ring may be used alone or in combination. Other variations of the device provide two opposing conical shaped attachments or a conical and flat ring attachment installed together to provide load capability in both axial and lateral directions while still providing angular flexibility.
U.S. Pat. No. 5,683,205, issued Nov. 4, 1997, to J. E. Halkyard, discloses a stress relieving joint for pipe such as risers, tendons, and the like used in floating vessel systems wherein a vessel is subject to heave, pitch, and roll motion caused by wind, currents, and wave action; the pipe passing through a constraint opening in the vessel and connected to the sea floor and subject to bending or rotation at the constraint opening. The joint comprises a sleeve member of selected length with ends at opposite sides of the constraint opening and centralizing annuli or rings at sleeve member ends for providing spaced contact points or areas to distribute bending stresses imparted to the sleeve member at the constraint opening to the pipe at the sleeve member ends.
U.S. Patent Application Publication 2002/0084077 A1, published Jul. 4, 2002, to Finn et al., discloses a spar type floating platform having risers passing vertically through the center well of a spar hull. A gimbaled table supported above the top of the spar hull is provided for supporting the risers. The table flexibly is supported by a plurality of non-linear springs attached to the top of the spar hull. The non-linear springs compliantly constrain the table rotationally so that the table is allowed a limited degree of rotational movement with respect to the spar hull in response to wind and current induced environmental loads. Larger capacity non-linear springs are located near the center of the table for supporting the majority of the riser tension, and smaller capacity non-linear springs are located near the perimeter of the table for controlling the rotational stiffness of the table. The riser support table comprises a grid of interconnected beams having openings therebetween through which the risers pass. The non-linear springs may take the form of elastomeric load pads or hydraulic cylinders, or a combination of both. The upper ends of the risers are supported from the table by riser tensioning hydraulic cylinders that may be individually actuated to adjust the tension in and length of the risers. Elastomeric flex units or ball-in-socket devices are disposed between the riser tensioning hydraulic cylinders and the table to permit rotational movement between the each riser and the table.
The above cited prior art does not disclose suitable means for prevention of axial slippage of centralizers in a stress joint which may be subject to substantial forces as well as to corrosion and/or galvanic action. Consequently, there remains a need to provide an improved centralizer system with improved centralizers and centralizer mountings that are not subject to the above problems. Those of skill in the art will appreciate the present invention, which addresses the above problems and other significant problems.